A variety of breathing assistance devices, which we will also generally refer to as “respirators” in this text, are available today.
These respirators are equipped with a source of respiratory pressurized gas. They are qualified as “autonomous” as an external pressurized gas feeding is not required to operate them.
These devices provide the patient, at each inspiration, with a respiratory gas (typically ambient air to which a complementary gas such as oxygen can be added).
Different types of respirators are known. These different types of respirators can be classified e.g. according to their size.
Indeed, the size of these devices is an important parameter: it is generally desirable to minimize this size, in order to facilitate the operation of a same and single device in varied places and circumstances (e.g. home, as well as hospital), and in order to increase the mobility of the patient.
Non-Transportable Devices
A first type of respirators relates to the ones qualified as being non-transportable. This first type is schematically illustrated in FIGS. 1a to 1d. 
Such devices are generally equipped with a respiratory gas source S1 having a very large size and/or weight. This gas source can be internal to the device, located in this case in a central unit 10, as the non-transportable respirator described hereinafter and illustrated in FIGS. 1a to 1d. The gas source can also be external to the device.
In these devices, the source of gas is coupled to the patient P through two ducts, an inspiration duct 11 dedicated to the inspiration phase and through which the patient P inspires the pressurized gas from the source of gas, and an expiration duct 12 dedicated to the expiration phase and through which the patient can exhale expiratory gases, such as carbon dioxide.
These non-transportable respirators are further provided with an inspiratory valve 13 and an expiratory valve 14. These two valves are located close to the gas source S1, respectively on the inspiration duct 11 and on the expiration duct 12.
The inspiratory valve 13 allows controlling the flux of the pressurized gas transmitted to the patient during the respiratory phases.
The expiratory valve 14 allows the expiratory gases of the patient to pass out of the expiratory duct 12, in the surrounding atmosphere. For this purpose, the expiratory valve can further be controlled with a PEP (Positive Expiratory Pressure).
Most of the operating modes of the respirators require a monitoring of the expiratory gas flow and/or expiratory pressure. Therefore sensor(s) 19 for sensing the gas flow and/or pressure have to be provided in the respirator.
Each sensor usually needs to be connected to the central unit 10 of the respirator by at least three wires, in order to be supplied with power and to convey data.
Therefore the sensors 19 are generally located near the gas source S1 in order to avoid further increasing the complexity of the already quite complex and large double transmission circuit by the addition of sensors and wires.
If it is desired that the sensors 19 are located in the vicinity of the expiratory valve, said expiratory valve 14 has thus to be located close to the gas source S1.
Both the inspiratory and expiratory valves require specific and often complex controlling means 15, i.e. controller 15, in order to be operated properly.
The non-transportable respirators are generally provided with relatively long ducts, of about 150 to 180 cm.
This configuration results in a high breathing resistance which increases the work of breathing of the patient.
Indeed, if the expiratory valve 14 is located at the end of the expiration duct 12 near the gas source S1 (distal end), and the expiration duct 12 being relatively long, the patient P will need to “push” his expiration through the expiration duct 12 until the expired air reaches the expiration valve to be vented to the atmosphere.
Transportable Respirators
A second type of respirators can be referred to as transportable respirators, as schematically illustrated in FIGS. 2a to 2d. This type of transportable respirator is provided with a central unit 20 comprising an internal respiratory gas source S2.
The gas source S2 may be a small turbine or blower, having optimized characteristics in order to limit the volume occupied by the device.
A further way to limit the volume of these devices is to use a single gas transmission duct 21 between the source S2 and the patient P, in contrast with devices having two ducts (an inspiration duct and an expiration duct).
The operation principle of these respirators is based on the use of an expiratory valve 22 located on the single duct 21, near the patient P (i.e. at the proximal end of the duct).
Such proximal localization of this expiratory valve 22 allows, in particular during the expiratory phase, to avoid the breathing resistance phenomenon which would be caused by the length of the duct used for expiration if the expiratory valve was located at the distal end of the duct.
In the known transportable respirators, such as represented in FIGS. 2a to 2d, this expiratory valve 22 is a pneumatic valve being operated thanks to a pressurized air feeding conduit 23, coupled with the respiratory gas source S2 (or to another source of pressure such as an independent microblower), and which inflates an obstructing cuff 24 of the expiratory valve 22.
Such control of the expiratory valve thus requires a specific conduit 23, which limits the miniaturization of the respirator.
During the expiration phase, the expiratory valve 24 is either opened or partially closed in order to establish a positive expiratory pressure (PEP) in the gas transmission duct to balance the residual overpressure in the patient lungs.
In order to establish such a PEP, it is necessary to control very precisely the pneumatic inflating pressure of the cuff 24 of the expiratory valve 22. This increases the complexity of the controller 25 of the respirator.
In some respiratory modes, the expiratory valve has to be operated as much as possible in real time, which is quite difficult in such expiratory valves because of the pneumatic inertias which are associated with them.
Moreover the configuration of such a known respirator imposes a limitation of the value of the PEP at around 20 mBar, while some respiratory modes would need a higher value of the PEP (e.g. 40 mBar or even more).
For the same reason as for non-transportable respirators, the expiratory gas flow and/or expiratory pressure may have to be controlled and gas flow and/or pressure sensors 29 have therefore to be provided near the expiratory valve 22.
Here again this requires providing wires along the gas transmission duct 21 between the central unit 20 containing the gas source S2 and the patient P (namely three wires—two for power supply and one for data transmission—for each pressure sensor, and two power supply wires for each gas flow sensor). Since expiratory gas flow and pressure generally have to be measured, a connection cable 26 of at least five wires is thus required between the central unit 20 and the expiratory valve 22 at the proximal end of the device.
Comment on Situation of Disabled Control of the Expiratory Valve
In order for the patient to safely use a respirator, the latter being transportable or not, this device must of course allow the patient to breathe in any situation, including if the pressurized gas source is disabled (breakdown or other). There are therefore safety standards to fulfil so that the breathing assistance device can work even if the gas source is disabled.
Thus, with a respirator having a single gas transmission duct 21 as described before and a specific conduit 23 for pneumatic control of the expiratory valve 22, the patient P can always expires through the pneumatic expiratory valve 22, even if the pneumatic feeding of the expiratory valve 22 is disabled, as shown in FIG. 2d. 
Indeed, if the pneumatic feeding of the expiratory valve is disabled, (this being the case when the gas source is disabled, if the source provides the control of the valve), the cuff 24 of the expiratory valve 22 will not be fed anymore, preventing therefore the PEP control, but still allowing the patient P to reject the expiratory gases E.sub.P through the expiratory valve 22.
In such case, it will however be impossible for the patient P to inspire through this pneumatic expiratory valve 22, since the cuff 24 shall obstruct the passage between the inside and the outside of the transmission duct 21, because of the patient inspiration IP.
Consequently, transportable respirators as illustrated in FIGS. 2a to 2d comprise a safety back flow stop valve 27 near the gas source S2. As represented in FIG. 2a, this safety valve 27 will normally be closed under the effect of the pressure feeding GS coming from the gas source S2, but if the latter is disabled, the pressure of the patient inspiration IP will open the safety valve 27, allowing the patient P to inspire air from outside, as illustrated in FIG. 2c. 
The disabling of the gas source S2 corresponds to a particular case of disabling of the pneumatic control of the expiratory valve 22. It is specified that in this text such disabling of the gas source S2 is understood as more generally referring to a disabling of the pneumatic control of the expiratory valve 22.
In order to allow a safe inspiration through the safety valve 27 and the whole length of the duct 21, the diameter of the duct will have to be large.
It is specified in this respect that there are generally pressure loss standard requirements to fulfil for addressing this issue of safety. For example, the French standards state that the maximum pressure loss between the source and the patient must not exceed 6 hPa for 1 liter•second for an adult and 6 hPa for 0.5 litre•second for a child.
And in order to fulfil such requirements, the transmission duct of known devices such as illustrated in FIGS. 2a to 2d must have a minimum diameter of 22 mm for an adult and a minimum diameter of 15 mm for a child.
Such large diameter of the duct is of course an obstacle to miniaturization of the device.
For a non transportable respirator (see FIGS. 1a to 1d), the patient P will always be able to expire through the expiration duct 12, even if the gas source S1 is disabled, as shown in FIG. 1d. 
If the gas source S1 is disabled, as illustrated in FIG. 1c, the inspiration phase is made possible through a safety back flow stop valve 16 located on the inspiration duct 11, near the gas source S1.
This safety back flow stop valve 16 is not located on the expiration duct 12 as it would be dangerous for the patient P to inspire through the expiratory duct 12 which contains a plug of carbon dioxide.
For the same reasons as for the transportable respirators, the diameters of the duct must be relatively large to fulfil the pressure loss requirements, that is a least 15 mm for children and 22 mm for adults, in order to allow a safe inspiration through the safety valve 16.
And here again, such large diameter is an obstacle to miniaturization.
Comment on Ability to Operate According to Different Modes
In addition, it is to be noted that the pathologies and diseases to be treated by the respirators are varied, and the breathing assistance devices can therefore be of different types, such as pressure-controlled or volumetric-controlled, and be operated according to different operating modes.
Each operating mode is defined by particular setting and checking variables but also by a particular type of material.
Some devices, which can be referred to as hybrid, are able to work according to several operating modes. However their material configuration, in particular the accessories (as the type of ducts between the gas source and the patient, the presence or not of an expiratory valve, the use of a mask with apertures, etc.), must be adapted to the chosen operating mode. And it would be desirable to operate a same and single device according to a large variety of modes, without requiring adapting the device (i.e. adapting its ducts, accessories, etc.).
Generally, it is an object of the invention to address one or more of the limitations and drawbacks mentioned above in this text.